A little explanation is in order with this one. The "0" (zero) after the % sign indicates a zero pading.The 3 indicates the "size" of the numeric pad. The "d" indicated numeric integer. The "\n" is newline.
The NR is an awk system variable for the current "record number." Thus my stipulation beforeabout the other "stuff" in the file, i.e. blank lines, non-printables, lines before and after the textual body, etc